1. Field of the Invention
The present invention relates to an equalizing circuit and an equalizer included in said equalizing circuit for compensation for distortion or other deterioration in the signal characteristics in a signal transmission line. More particularly, the present invention relates to an equalizing circuit adapted to be provided in a signal transmission line for transmitting signals of a time division multiplex access system or the like in a time dividing manner.
2. Description of the Prior Art
FIG. 1 is a block diagram showing an example of the construction of a conventional equalizing circuit 12 provided in a time division multiplex access (TDMA) system for allotting the time for transmission and reception to the respective associated regions in case of a satellite broadcast and the like. In FIG. 1, an input terminal 1 is connected to a distributor 3. The distributor 3 distributes the input signal to a desired number of paths. Output terminals of the distributor 3 are connected with circuits each comprising an equalizer 4 and a hopping switch 5 connected in series. In FIG. 1, an equalizer and a hopping switch in each path are denoted by reference numerals 4 and 5, respectively, plus characters a, b or c after the numerals for identifying each path. The hopping switches 5a, 5b and 5c are selectively enabled by a selection signal applied from a control input terminal 6. Accordingly, prescribed signals are obtained at the output terminals 2a, 2b and 2c in a time dividing manner.
FIG. 2 is a schematic block diagram of the whole construction of a TDMA system for explaining the operation state of the equalizing circuit 12 in FIG. 1 in the TDMA system. Referring to FIG. 2, the operation of the whole TDMA system will be briefly described.
A TDMA common terminal equipment 11 changes a plurality of signals to be transmitted to pulse signals and sends those pulse signals with a fixed cycle according to a reference timing signal. The equalizing circuit 12 distributes the pulse signals supplied thereto serially from the TDMA common terminal equipment 11 to a plurality of paths according to the kinds of the signals. At this time, the equalizing circuit 12 equalizes the distortions in the amplitude and the group delay of the signals so that the respective signals may be adapted for the corresponding paths.
The pulse signals distributed to the respective paths are transmitted from the transmitters 13a, 13b and 13c to the respective destinations, namely, an X station 15a, a Y station l5b, and a Z station l5c through transponders 14a, l4b and l4c provided in a communication satellite corresponding to the respective paths.
FIG. 3 is a timing chart of signals for explaining the operation of the equalizing circuit 12 in FIG. 1. Referring to FIG. 3, the operation of the equalizing circuit 12 in FIG. 1 will be specifically described.
The signal applied to the input terminal 1 is as shown in FIG. 3 (b). This signal is supplied with a fixed cycle according to the reference timing signal 16 shown in FIG. 3 (a) and is distributed through the distributor 3 to three paths so that the signals shown in FIG. 3 (b) are outputted from the respective output terminals. The equalizers 4a, 4b and 4c change the characteristics of the signals for the respective paths based on predetermined coefficients. The hopping switches 5a, 5b and 5c connected to the equalizers 4a, 4b and 4c, respectively, are selectively enabled so that only the signals necessary for the respective paths are extracted based on the hopping control signal supplied from the TDMA common terminal station 11 (as shown in FIG. 2) through a control signal input terminal 6. As a result, signals as shown in FIG. 3 (c), 3 (d) and 3(e) are outputted from the output terminals 2a, 2b and 2c, respectively.
The above stated equalizing circuit 12 in the TDMA system functions to equalize the signal transmission characteristics in the respective signal paths, such as the amplitude frequency response characteristics or the group delay frequency response characteristics in the respective paths. However, the amplitude frequency response characteristics and the group delay frequency response characteristics are different for each signal path, in other words, for each transmitter 13 or each transponder 14. For this reason, it is necessary to connect an equalizer for each signal path and to make equalization so that the signal characteristics adapted for each signal path can be obtained. Consequently, the conventional equalizing circuit is required to have equalizers the number of which is equal to the paths for signals to be transmitted and there is posed a problem because it is necessary to increase the number of the equalizers according to the increase of the number of the signal paths.